The effect of substituents in PNPN=H ligands on activity and selectivity of a chromium-based catalyst system for highly selective ethylene trimerization

Article abstract of DOI:10.1002/cctc.201300324

The influence of ligand substituents on a highly selective homogeneous catalyst system for the trimerization of ethylene to 1-hexene consisting of chromium(III) acetylacetonate, a ligand with a PNPN=H backbone, tetraphenylphosphonium chloride as chlorine source, and triethyl aluminum as activator was investigated. These N-modified and P-modified ligands, which differ strongly in their electronic and steric properties of the substituents, were tested in the ethylene trimerization at three temperatures (50, 70, and 90°C). Interestingly, the substituents at the N atoms of the PNPN=H-backbone had only little effect on the catalytic activity, on 1-hexene selectivity, and on thermal stability of the catalyst system, whereas the substituents at the P atoms significantly influenced the efficiency of the system. This is even more surprising as the PNP ligand from SASOL for ethylene tetramerization is much more sensitive against substituent variation at its N atom. A family competition between selected ligands with a PNPN=H motive turns out to be a neck-and-neck race. A highly selective trimerization catalyst for ethylene is optimized through fine-tuning of structural and electronic properties.

Full text of DOI:10.1002/cctc.201300324

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A family competition between selected
ligands with a PNPNꢀH motive turns
out to be a neck-and-neck race. A
highly selective trimerization catalyst for
ethylene is optimized through fine-
tuning of structural and electronic
properties.
S. Heinig, A. Wçhl, W. Mꢀller,*
M. H. Al-Hazmi, B. H. Mꢀller, N. Peulecke,
U. Rosenthal*
&& – &&
The Effect of Substituents in PNPNꢀH
Ligands on Activity and Selectivity of
a Chromium-Based Catalyst System
for Highly Selective Ethylene
Trimerization
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DOI: 10.1002/cctc.201300324
The Effect of Substituents in PNPNꢀH Ligands on Activity
and Selectivity of a Chromium-Based Catalyst System for
Highly Selective Ethylene Trimerization
Stefan Heinig,[a] Anina Wçhl,[a] Wolfgang Mꢁller,*[a] Mohammed H. Al-Hazmi,[b]
Bernd H. Mꢁller,[c] Normen Peulecke,[c] and Uwe Rosenthal*[c]
The influence of ligand substituents on a highly selective ho-
mogeneous catalyst system for the trimerization of ethylene to
1-hexene consisting of chromium(III) acetylacetonate, a ligand
with a PNPNꢀH backbone, tetraphenylphosphonium chloride
as chlorine source, and triethyl aluminum as activator was in-
vestigated. These N-modified and P-modified ligands, which
differ strongly in their electronic and steric properties of the
substituents, were tested in the ethylene trimerization at three
temperatures (50, 70, and 908C). Interestingly, the substituents
at the N atoms of the PNPNꢀH-backbone had only little effect
on the catalytic activity, on 1-hexene selectivity, and on ther-
mal stability of the catalyst system, whereas the substituents
at the P atoms significantly influenced the efficiency of the
system. This is even more surprising as the PNP ligand from
SASOL for ethylene tetramerization is much more sensitive
against substituent variation at its N atom.
Introduction
Linear a-olefins are versatile intermediates for the petrochemi-
cal industry. Their application is strongly dependent on their
chain length. Whereas 1-butene, 1-hexene, and 1-octene are
used as comonomers in the production of polyethylene (HDPE,
LLDPE), longer chain olefins are converted to surfactants, plas-
ticizers, or lubricants.[1] Each market segment has a different
size and growth rate. Thus, it is difficult to cover the market
with the conventional full-range processes, such as the Shell
Higher Olefin Process or the a-Sablin technology.[2] The co-
monomer market segment has the highest growth rate with
5–6%/a, whereas the market for long-chain olefins is expected
to grow only by 3% per annum in the years 2009–2014.[1]
Owing to the market imbalance, the interest in selective routes
leading to the high-value comonomer fractions (C6 and C8) has
increased drastically. In recent years, a multitude of different
ethylene tri- and tetramerization catalyst systems have been
published.[3] The main focus has been on homogeneous chro-
mium-based catalyst systems, often in combination with mixed
P and N donors and an aluminum alkyl (e.g., TEA:triethyl alu-
minum and MAO:methyl aluminoxane) as activator.[3]
In previous publications by our group, the utilization of
novel PNPNꢀH ligands in a chromium-based catalyst system
has been described, which converts ethylene to comonomer-
grade 1-hexene with more than 90% yield.[4] For the ligand
Ph2PN(iPr)P(Ph)N(iPr)ꢀH (L0), we reported on detailed kinetic
data and modeling studies,[5] investigated its coordination be-
havior,[6] and conducted organometallic studies with relevance
to activation and deactivation of the catalyst system[7] as well
as its heterogenization.[8] Some disadvantages such as the
comparatively low catalytic activity (4–8 kggCrꢀ1 hꢀ1) of the
original system, which was based on CrCl3(THF)3 as chromium
precursor, could be eliminated by these modifications.[9] The
modified catalyst system, which consisted of the chlorine-free
chromium(III) acetylacetonate, Cr(acac)3, instead of the expen-
sive and sensitive CrCl3(THF)3, reached activities of 40–
60 kggCrꢀ1 hꢀ1 if combined with an additional chlorine source
(e.g., ammonium or phosphonium chlorides), the PNPNꢀH
ligand, and TEA. Depending on the precise reaction conditions,
this modification led to an approximately tenfold increase in
activity over that of the previous system.
[a] S. Heinig, Dr. A. Wçhl, Dr. W. Mꢀller
Linde AG, Linde Engineering Division
Dr.-Carl-von-Linde-Str. 6-14, 82049 Pullach (Germany)
Fax: (+49)8974454981
Based on these earlier results, our objective was to investi-
gate the influence of various N and P substituents of the
ligand on the activity and selectivity of the catalyst during the
oligomerization reaction. In a recent publication,[10] we report-
ed on the preparation and characterization of several ligands,
which were now tested in catalysis with the objective to assess
the effects of steric and/or electronic changes in the ligand
structure on the activity, selectivity, and thermal stability. The
ten ligands investigated were divided into two groups: first,
the N-modified ligands and, second, the P-modified ligands
(Figure 1).
E-mail: Wolfgang.mueller@linde-le.com
[b] Dr. M. H. Al-Hazmi
Saudi Basic Industries Corporation,
P.O. Box 42503, Riyadh 11551 (Saudi Arabia)
[c] Dr. B. H. Mꢀller, Dr. N. Peulecke, Prof. Dr. U. Rosenthal
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock e.V.
Albert-Einstein-Str. 29A, 18059 Rostock (Germany)
Fax: (+49)381128151176
E-mail: Uwe.rosenthal@catalysis.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300324.
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Table 1. Average activities and selectivities of the trimerization system
Cr(acac)3/LX/[Ph4P]Cl/AlEt3. Runs were conducted at 508C, 30 bar, [Cr]=
0.3 or 1.0 mmolLꢀ1, t=1 h, [L]/[Cr]=1.3, [Al]/[Cr]=25, [Cl]/[Cr]=5; all
ratios in molar units.
Ligand
Activity[a]
[kggCrꢀ1 hꢀ1
Selectivity [wt%]
]
C4
C6
1-C6
C8
C10
4.2
PE
L0
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
44.9
44.5
42.0
38.0
42.1
21.7
3.1
36.2
14.2
3.2
2.5
2.1
2.9
3.1
3.0
7.4
3.8
4.2
3.4
2.9
4.0
92.5
93.1
92.1
92.4
87.3
86.8
88.3
86.8
88.6
62.3
85.9
99.0
99.0
98.9
99.0
98.9
98.7
96.8
98.8
97.8
95.4
98.7
0.4
0.3
0.1
0.1
0.1
0.1
0.4
4.5
0.2
0.6
3.9
0.3
0.4
0.4
0.4
0.8
1.8
2.7
0.9
1.4
13.1
1.1
4.3
4.0
3.6
8.5
3.4
3.5
7.6
6.3
18.3
8.6
34.1
[a] Average activity over 1 h.
N-modified ligands were approximately as high as those of the
original ligand L0, whereas modifications at the P atoms led to
dramatic drops in activity.1 Nevertheless, all investigated li-
gands with the PNPNꢀH backbone (except ligand L9) exhibited
a selectivity >85 wt% towards 1-hexene. 1-Hexene selectivities
of more than 90 wt% could be obtained with ligands L0, L1,
L2, and L3, with 1-butene selectivities ranging between 2.5
and 3.1 wt%. Notably, the C10 fraction consisted of several
branched C10 isomers, indicating a formation mechanism that
involves the incorporation of a 1-hexene molecule into the
metallacycle. Only the ligand L9 with cyclopentyl groups at
the terminal P atom exhibited a relatively poor selectivity of
>60 wt%. From the GC pattern it was suggested that a nonse-
lective catalyst was formed through decomposition of the
ligand, producing an underlying Schulz–Flory distribution. This
might be also the reason for the enhanced C8 formation in this
case. In addition, the relative amount of formed polymer in-
creased if the P atoms were modified and no phenyl substitu-
ents stabilized the phosphorus atom.
Figure 1. Structures of all investigated N-modified ligands (clear). Structures
of all investigated P-modified ligands (blue).
Results and Discussion
All ligands (L0–L10) were investigated at 50, 70, and 908C,
whereas all other parameters were kept constant. The objec-
tive was to examine their thermal deactivation behavior and to
determine typical catalyst performance parameters such as se-
lectivity, 1-hexene purity (i.e., 1-hexene selectivity with respect
to the total C6 fraction), and activity. The temperatures were
chosen based on previous studies,[5] which showed that the
original catalyst system with ligand L0 starts to deactivate at
elevated temperatures (>608C). This deactivation is most likely
associated with ligand deprotonation and subsequent rear-
rangement of the ligand in the presence of the co-catalyst
TEA.[7] The evaluation of the results for ligands with varying
substituents can give information on the deactivation of the
respective catalyst.
A similar behavior was observed if the 1-hexene selectivity
in the C6 fraction, that is, the 1-hexene purity, was considered.
Although all N-modified ligands exhibited purities of approxi-
mately 99 wt%, the P-modified ligands (L6, L8, L9) exhibited
significantly reduced purities down to 95.4 wt%.
The 1-hexene purity is of central importance for every tech-
nical-scale processes using one of these catalyst systems. The
reason for this is the necessity to achieve very high 1-hexene
purities through the intrinsic chemical properties of the
system, so as to avoid any need for a superfractionation of the
C6 cut.
Regarding the ethylene uptake curve in Figure 2, the experi-
ments with ligands L0–L5, which were all modified at the N-
atoms, exhibited approximately constant ethylene consump-
Ligand screening at optimized reaction conditions
1
For simplicity, we refer here to the “activity of the ligand” meaning, of course,
All ligands (N- and P-modified) were first tested under previ-
ously optimized standard conditions at 508C (Table 1). Interest-
ingly, the average activity was strongly affected by the location
of the substituent modification. For example, the activities of
the activity of the respective catalyst system using this particular ligand and
comprising all necessary catalyst components, namely Cr source, ligand,
modifier, and activator. The same holds for other catalytic key figures such as
catalyst stability, selectivity, and product purity.
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stituents and the position of the N atom, that is, internal or ter-
minal, seems to influence the activity reduction verifiably. The
P-modified ligands L6, L8 and L9 all exhibited very low ethyl-
ene consumption rates, suggesting that they were much less
active than the N-modified ligands. Owing to the low activity,
the P-modified ligands were investigated at a higher chromium
concentration of 1.0 mmolLꢀ1. The results are discussed below.
Thermal stability of the N-modified ligands
Already at a reaction temperature of 708C a thermal deactiva-
tion of the catalyst system could be observed for all ligands,
induced by ligand deprotonation and subsequent rearrange-
ment.[7] This was clarified by a strong curvature of the ethylene
uptake curves as exemplified by the curves of ligand L1 with
cyclohexyl groups at both N atoms (Figure 3). After a short ini-
Figure 2. Ethylene uptake curves of all investigated ligands of the trimeriza-
tion system Cr(acac)3/LX/[Ph4P]Cl/AlEt3. Runs were conducted at 508C,
30 bar, [Cr]=0.3 mmolLꢀ1, t=1 h, [L]/[Cr]=1.3, [Al]/[Cr]=25, [Cl]/[Cr]=5.
tion over time, meaning that the catalyst was not deactivated.
As a result, a shift from the isopropyl (L0) to the cyclohexyl
group at one of the N atoms (L2 and L3) or at both N atoms
(L1) had only a minor influence on the catalytic performance,
although the cyclohexyl substituent was more bulky. Neverthe-
less, the spatial demand was on a similar level for isopropyl
and cyclohexyl.
Interestingly, the ligand with the substituent (EtO)3Si(CH2)3 at
the terminal N atom (L4) exhibited an ethylene uptake that
was similar to that of the standard ligand, although this cata-
lyst system started to deactivate earlier. Owing to the fact that
a higher amount of branched decenes was formed with this
ligand, one can conclude that this system was more suscepti-
ble to incorporation of 1-hexene into the metallacycle. This led
to a lower ethylene consumption rate caused by high 1-
hexene concentrations in the reaction liquid, thereby blocking
the active center of the catalyst. In contrast, the catalyst sys-
tems with the ligands L5, L7, and L10, having n-alkyl substitu-
ents at one N atom, exhibited uptake curves with a detectable
curvature after short reaction times, indicating an additional
deactivation process or an increased susceptibility of the cata-
lyst complex to product inhibition. Although thermal deactiva-
tion at 508C by ligand deprotonation and subsequent rear-
rangement was unlikely, it could not be excluded that the
change in ligand structure shifted the deactivation
temperature.
Figure 3. Ethylene consumption of the trimerization system Cr(acac)3/
L1/[Ph4P]Cl/AlEt3 at different temperatures. Runs conducted at 50/70/908C,
30 bar, [Cr]=0.3 mmolLꢀ1, t=1 h, [L]/[Cr]=1.3, [Al]/[Cr]=25, [Cl]/[Cr]=5.
tial phase in which the reaction rate was faster as a conse-
quence of the elevated temperature (Arrhenius behavior), the
thermal deactivation predominated and slowed down the re-
action rate, an effect that was even more pronounced at 908C.
Clearly, a purely qualitative interpretation of the temporal
deactivation characteristics as shown, for example, for the
708C curve in Figure 3 could, in principle, lead to an entirely
different conclusion regarding the deactivation mechanism.
This alternative interpretation would hint at an enhanced 1-
hexene incorporation into the metallacycle as a competing
mechanistic pathway to the incorporation of ethylene, thereby
slowing down the overall reaction rate and leading to a higher
amount of long-chain oligomers such as C10. However, taking
the full picture from parameter studies and kinetic modeling
work into consideration, this mechanism can largely be ruled
out.[5] Parameter studies with varying temperatures and other-
wise constant conditions showed clearly that a high 1-hexene
selectivity and a constantly low selectivity to C10 isomers was
maintained throughout the temperature regime of the investi-
gation (50–908C, additional experiments given in the Support-
ing Information). Consequently, the observed deactivation
could not be attributed to product inhibition. Moreover, kinetic
In the case of an n-hexyl group at the internal N atom (L5),
the activity was halved relative to that of the standard ligand
(L0). The long freely rotatable alkyl chain seemed to inhibit the
reaction at the catalytic center.
In contrast, short alkyl substituents (ethyl and methyl) at the
terminal N atom (L7 and L10) resulted only in a minor de-
crease in activity compared to the isopropyl group. Clearly, the
reduced space requirements of the methyl and ethyl groups
relative to that of the isopropyl group of the standard ligand
induced no significant positive effect on the catalyst perfor-
mance. These results confirm the assumption that small
changes in the ligand structure, for example, an expansion of
an alkyl group by one methylene group, do not influence the
catalytic behavior significantly. However, long-chain alkyl sub-
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modeling work for this type of catalytic oligomerization reac-
tion indicated that the observed deactivation time characteris-
tics could not be explained by either irreversible or reversible
product inhibition. This, in turn, was strongly supported by the
observation of elevated levels of 1-hexene having no signifi-
cant influence on the productivity.[5,7]
Consequently, the following sequence for the center N-atom
could be derived:
cyclohexyl ðL3Þ > hexyl ðL5Þ > isopropyl ðL0Þ
However, exchanging both substituents of the N atoms by
cyclohexyl substituents resulted in a faster deactivation of the
catalyst system.
This latter observation is a key factor for the transfer of
these catalyst systems into a viable technology, because con-
tinuous technical-scale process based on these systems has to
tolerate very high steady-state 1-hexene levels in the reaction
phase without discernible deterioration of the productivity.
The degree of thermal deactivation differed slightly with dif-
ferent substituents, which became clear for the average activi-
ties (determined after 1 h) at different temperatures in
Figure 4.
Thermal stability of the P-modified ligands
Owing to the low activity of the three tested P-modified li-
gands, the chromium concentration was increased to achieve
higher ethylene consumption rates at a residence time of 1 h.
Already at 508C, the ethylene uptake behavior and, hence, the
catalytic performance of the catalyst systems differed with dif-
ferent P substituents (Figure 5). For example, the ligand L8
Figure 4. Average activities of the trimerization system Cr(acac)3/LX/[Ph4P]Cl/
AlEt3 at different temperatures with different N-modified ligands. Runs con-
ducted at 50/70/908C, 30 bar, [Cr]=0.3 mmolLꢀ1, t=1 h, [L]/[Cr]=1.3, [Al]/
[Cr]=25, [Cl]/[Cr]=5.
Figure 5. Ethylene consumption of the trimerization system Cr(acac)3/
L6, L8, L9/[Ph4P]Cl/AlEt3 at 508C. Runs conducted at 508C, 30 bar,
[Cr]=1.0 mmolLꢀ11, t=1 h, [L]/[Cr]=1.3, [Al]/[Cr]=25, [Cl]/[Cr]=5.
In the case of the cyclohexyl group at the terminal N atom
(L2), a similar temperature-dependent behavior was observed
to that of the ligand with isopropyl (L0) substituent. The sub-
stituent (EtO)3Si(CH2)3ꢀ in L4 reduced the thermal stability of
the catalyst complex already at 708C. Either the space require-
ment of this substituent influenced the stability negatively or
the ligand had a higher tendency to decompose. As the Si sub-
stituent is very sensitive, it is possibly decomposed or attacked
by TEA at high temperatures. In contrast, if the substituent at
the terminal atom was replaced by a short alkyl group (L7 and
L10), the thermal stability was slightly enhanced. In conclusion,
the following thermal stability sequence for the substituents at
the terminal N atom (isopropyl substituent at the center N
atom) was proposed:
with a methyl group instead of a phenyl group at the internal
P atom exhibited the highest ethylene uptake rate of the three
P-modified ligands, but at a significantly reduced activity com-
pared to that of the standard L0. However, the distinct shape
of the ethylene uptake curve, indicating a deactivation process
of L8 at this temperature, was surprising. Clearly, the smaller
size (steric effect) and the different electronic structure of the
methyl group was responsible for this behavior.
Modifications of the substituents at their terminal P atom
(L6 and L9) led to similar ethylene consumption characteristics
at a low constant reaction rate. Moreover, the induction
period, that is, the time between ethylene pressurization and
start of ethylene consumption, was considerably pronounced
with these two ligands.
methyl ðL10Þ > ethyl ðL7Þ > isopropyl ðL0Þ, cyclohexyl >
ðEtOÞ3SiðCH2Þ3ꢀ ðL4Þ
The low activity and the poor selectivity caused by the
ligand L9, carrying two cyclopentyl rings, was somehow sur-
prising because these rings had a similar size to the phenyl
rings of the standard ligand L0. Therefore, this result confirms
the hypothesis that the phenyl substituents with their p-elec-
tron system are necessary to stabilize the active catalysts.[11]
The phenyl substituents increased the electron density of the
chromium catalyst complex, resulting in a catalytically more
active complex. It is also interesting that the heterocycle of L6
For the center N atom, a slightly enhanced thermal stability
was observed with the cyclohexyl substituent. The ligand L5
with the hexyl group, which already exhibited a relatively low
activity at 508C, had a higher average activity at 708C. At 908C
its activity was similar to that of L3, which means that this cat-
alyst system decomposed not as fast as the other systems.
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did not decrease the activity more than the cyclopentyl
groups, although the substituent was not freely rotatable.
However, it is also possible that additional ligand substituent
effects may influence the catalytic performance. Especially the
changed spatial demand of the substituents and the changed
donor properties of the P atoms have a minor influence on the
performance of the catalytic species.[3e]
measured also at 1108C. In the following the behavior of typi-
cal catalyst performance parameters as a function of tempera-
ture is shown. In Figure 7 the change of the product distribu-
tion (selectivities) with temperature is visualized.
In the case of increased temperature regimes (70 and 908C),
all catalyst systems exhibited approximately the same or en-
hanced average activities compared to those at 508C
(Figure 6). This suggests that the P-modified ligands are much
Figure 7. Product distribution of the trimerization system Cr(acac)3/
L7/[Ph4P]Cl/AlEt3 at different temperatures. Runs were conducted at 50/70/
90/1108C, 30 bar, [Cr]=0.3 mmolLꢀ1, t=1 h, [L]/[Cr]=1.3, [Al]/[Cr]=25,
[Cl]/[Cr]=5.
The formation of all main and side products are coupled
through the metallacycle mechanism[12] and each of these reac-
tions has its own rate constant with an individual temperature
dependency. The proportions of the C4 fraction (mainly 1-
butene) and the C6 fraction (mainly 1-hexene) were almost
constant at temperatures between 50 and 908C, which is
a unique feature of ligands L7 and L10. All other ligands exhib-
ited a significantly changed product composition already at
908C (compared with 508C/708C, see the Supporting Informa-
tion). At 1108C, the C6 selectivity was reduced by approximate-
ly 10 wt% and the C4 selectivity was increased by approxi-
mately 2.6 wt%, whereas simultaneously the amount of the C8
(up to 10 wt%) and C12 fractions was significantly increased.
The higher formation rate of 1-butene can be explained by
a preferred b-hydride elimination of 1-butene from the metal-
lacyclopentane complexes at higher temperatures. But it is
also possible that through thermal decomposition of the cata-
lyst system new active species were formed, leading to a non-
selective production of linear a-olefins. This would lead to a su-
perposition of a Schulz–Flory distribution and the usual selec-
tive trimerization product distribution.
Figure 6. Average activities of the trimerization system Cr(acac)3/LX/[Ph4P]Cl/
AlEt3 at different temperatures with different P-modified ligands. Runs con-
ducted at 50/70/908C, 30 bar, [Cr]=1.0 mmolLꢀ1, t=1 h, [L]/[Cr]=1.3,
[Al]/[Cr]=25, [Cl]/[Cr]=5.
more temperature-stable than their N-modified counterparts,
despite their comparatively low activity. However, ligand L9
with cyclopentyl substituents was more sensitive to the tem-
perature increase than L6 with the heterocycle. Interestingly,
the kinetic behavior of L8 changed at higher temperatures
(see the Supporting Information). The ethylene consumption
was nearly linear at 708C and had no curvature as it was ob-
served at 508C. At 908C, a deactivation process could be ob-
served. An explanation of this effect was not possible without
further investigations.
In general, small substituents such as methyl (L8) or isopro-
pyl (L6) instead of bulky substituents such as phenyl (L0) or cy-
clopentyl (L9) at one of the two P atoms led to the observed
deactivation at 508C. Further investigations are necessary to
understand the low-temperature deactivation effect. Further-
more, the P-modified L8 reached the activity level of the N-
modified ligands (activity level of L0) at 908C. It is therefore
proposed that each variation of the substituents at the P
atoms of the PNPNꢀH backbone significantly reduces the activ-
ity of the catalyst system. Furthermore, the phenyl substituents
at both P atoms of the standard ligand were identified as im-
portant structural function blocks for high catalyst
performances.
In Figure 8 the important parameters such as average activi-
ty, 1-hexene purity, and polymer formation are summarized as
a function of temperature. As discussed above, ligand L7 ex-
hibited the highest thermal stability of all tested ligands. The
catalyst system using L7 was still active at 1108C. Presumably,
the ligand substituent of L7 and also of L10 prevented or, at
least, slowed down the rearrangement of the ligand at high
temperatures in the catalyst complex. It is also possible that
the rearranged complex had a better activity than the complex
of the other ligands.
Detailed thermal behavior of L7
Ligand L7 (and also L10) exhibited a higher thermal stability
than the other investigated ligands. Therefore, the ligand was
The purity of the C6 fraction decreased with increasing tem-
perature. It can be concluded that higher temperatures lead to
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Figure 8. Average activity, 1-hexene purity, overall 1-hexene selectivity, and polymer formation depending on temperature of the trimerization system Cr-
(acac)3/L7/[Ph4P]Cl/AlEt3. Runs were conducted at 50/70/90/1108C, 30 bar, [Cr]=0.3 mmolLꢀ1, t=1 h, [L]/[Cr]=1.3, [Al]/[Cr]=25, [Cl]/[Cr]=5.
the formation of more hexene isomers. This can be traced
back to an acceleration of isomerization reactions at higher
temperatures in the presence of AlEt3.
complex. Nonaromatic substituents, in contrast, reduce the ac-
tivity significantly.
The polymer-to-product ratio was nearly constant at temper-
atures between 50 and 708C. At higher temperatures, the poly-
mer fraction was strongly increased up to 2 wt%. It appears
that elevated temperatures supported the polymer formation
by an insertion mechanism. Another possibility is that the cata-
lyst complex was partially decomposed under release of a poly-
merization catalyst (TEA+“naked” chromium).
Conclusions
It is desirable to investigate a larger variety of substituents at
the inner N atom to allow for more unique conclusions regard-
ing the relative importance of the substitution variation at the
N versus the P atoms. Furthermore, ligands with identical sub-
stituents at both N atoms could be investigated to extend the
scope of possible conclusions regarding individual influences.
Unfortunately, we were not able to synthesize and/or char-
acterize all of the desired compounds with further N substitu-
ents that could introduce larger electronic changes. However,
keeping in mind that even very small changes in the N-atom
substitution of Sasol’s diphosphinoamine (PNP) system causes
dramatic effects, it appears remarkable that our current PNPNꢀ
H system was by far less susceptible to analogous variations at
the central N atom.[13b]
Comparison of substituent influences in diphosphinoamine
and PNPNꢀH ligands
A direct comparison of the effects of substituents on the cata-
lytic performance to other similar systems is considerably diffi-
cult. Only for the distantly related Sasol ethylene tetrameriza-
tion system, which uses diphosphinoamine (PNP) ligands, sev-
eral studies with various substituents at the PNP ligands at
two different sets of reaction conditions were conducted.[13] In
these studies, different substituents at the N atom and differ-
ent aromatic substituents at the P-atoms were tested. In gen-
eral, activity and selectivity seemed to be strongly dependent
on the ligand substituents especially at the N atom.[13] This de-
pendency was by far not as pronounced for the PNPNꢀH
ligand in our study.
Furthermore, the range of investigated ligand modifications
allowed for a comparison with the standard, or “reference”,
ligand L0 with phenyl substituents at the P atoms.
Therefore, for the restricted number of ligands investigated
in this study, one can conclude that the phenyl groups at the
P atoms are essential for a high catalytic activity. In addition,
the side-product selectivities were less temperature-sensitive if
phenyl substituents were bound to the P atoms. Furthermore,
comparison with L0 revealed that the substitution of the iso-
propyl at the terminal N atom by a methyl or ethyl group im-
proved the thermal stability significantly. Moreover, a-branched
(isopropyl or cyclohexyl) substituents at the N atoms promoted
Furthermore, an extension of the p-electron system using
naphthyl and biphenyl substituents resulted in a profound in-
crease of the activity relative to that with phenyl
substituents.[13a] This result confirmed the assumption that p-
electron systems of aromatic substituents activate the catalyst
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ChemCatChem 0000, 00, 1 – 8
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CHEMCATCHEM
FULL PAPERS
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[4] a) S. Peitz, N. Peulecke, B. R. Aluri, S. Hansen, B. H. Mꢁller, A. Spannen-
berg, U. Rosenthal, M. H. Al-Hazmi, F. M. Mosa, A. Wçhl, W. Mꢁller, Eur. J.
Inorg. Chem. 2010, 1167–1171; b) P. M. Fritz, H. Bçlt, A. Wçhl, W. Mꢁller,
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Mꢁller, M. H. Al-Hazmi, V. O. Aliyev, F. M. Mosa, (Linde AG/SABIC), WO
2009/006979 A2, 2009.
[5] a) A. Wçhl, W. Mꢁller, S. Peitz, N. Peulecke, B. R. Aluri, B. H. Mꢁller, D.
Heller, U. Rosenthal, M. H. Al-Hazmi, F. M. Mosa, Chem. Eur. J. 2010, 16,
7833–7842; b) W. Mꢁller, A. Wçhl, S. Peitz, N. Peulecke, B. R. Aluri, B. H.
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high 1-hexene selectivities, whereas the ligands were less tem-
perature-stable. Long alkyl chains such as n-hexyl at the inter-
nal N-atom led to a reduction of activity and to high amounts
of C4 (508C).
In summary, it was very surprising that the ethylene trimeri-
zation system based on the PNPNꢀH ligand was less sensitive
towards variation of substituents at the N atom of the ligand
than Sasol’s PNP tetramerization system. Consequently, it is
very unlikely to achieve an extraordinary activity increase or
significant change of the selectivity only by variation of N
substituents.
[6] B. R. Aluri, N. Peulecke, S. Peitz, A. Spannenberg, B. H. Mꢁller, S. Schulz,
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It is still questionable if the general kinetic behavior of the
catalyst system can be influenced by variation of the ligand
substituents. This pertains to, for example, the influence of the
catalyst composition (Al/Cr ratio, ligand/chromium ratio) or
pressure effects on the overall kinetics. These aspects will be
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[7] a) S. Peitz, N. Peulecke, B. R. Aluri, B. H. Mꢁller, A. Spannenberg, U.
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Experimental Section
A detailed description of the experimental setup, the catalyst prep-
aration and additional experimental results are given in the Sup-
porting Information.
[8] N. Peulecke, B. H. Mꢁller, S. Peitz, B. R. Aluri, U. Rosenthal, A. Wçhl, W.
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Acknowledgements
The authors thank Linde Engineering and SABIC for the permis-
sion to publish this work. Furthermore, we wish to thank Linde
Engineering’s R&D department for their valuable contributions as
well as the analytical service and the technical staff at LIKAT.
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Keywords: homogeneous catalysis
ligands · oligomerization · substituent effects
· ligand design · N,P
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Received: April 29, 2013
Revised: June 12, 2013
Published online on && &&, 0000
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 8
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8
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These are not the final page numbers!